The present invention relates to a new and improved method of, and apparatus for, correcting coincidence errors during the counting of two types of particles.
In its more particular aspects, the present invention is concerned with a new and improved method of, and apparatus for, the correction of coincidence errors during the counting of particles of a first type in the presence of particles of a second type in a mixed or hybrid suspension containing both types of particles. The particles of the second type are larger than and present in a higher concentration than the particles of the first type. For instance, the invention is especially concerned with the counting of thrombocytes in a diluted blood sample which contains therein erythrocytes in a much higher concentration than the thrombocytes.
A method and apparatus for counting of thrombocytes in the presence of erythrocytes have been disclosed, for instance, in German Patent Publication No. 3,020,263. According to the teachings of such patent the particles of a diluted blood sample generate electrical pulses in an analyzer, the amplitude of which pulses corresponds to the particle size. The pulse amplitude is compared with a threshold value which is set such that the smaller thrombocyte pulses can be distinguished from the much larger erythrocyte pulses. Both types of pulses are separately counted. The result of the thrombocyte-count is reduced by subtraction of a correction element, in order to correct the error caused by the co-counted erythrocyte particles. The correction element which is to be subtracted is a polynomial function of the result of the erythrocyte-count.
As has been found such type of correction fulfils the purpose of eliminating from the result of the thrombocyte-count the errors caused by the too small erythrocyte pulses. The aforementioned too small erythrocyte pulses, which are falsely counted as thrombocyte pulses, are caused whenever an erythrocyte particle passes through the measuring aperture or opening of the analyzer in an abnormal or irregular manner, for instance extremely slowly or along an extremely eccentric path of travel. The frequency of occurrence of such abnormal or irregular pulses at least approximately corresponds to a polynomial function of the number of erythrocytes or erythrocyte particles, and therefore, it should be readily understood that there must be subtracted from the uncorrected count result the corresponding false counts.
However, with this particle counting method and apparatus there is absolutely not corrected a different type of error which arises, namely the so-called cross-coincidence error which is caused during the simultaneous presence or coincidence of thrombocyte particles with an erythrocyte particle in the measuring aperture or opening. With such type of errors the smaller thrombocyte pulse or pulses are masked by the much larger erythrocyte pulse.